An agile filter is a filter whose center frequency can be voluntary shifted by electrical or electronic control means over a range of frequencies that may cover a large fraction of an octave. In association with frequency synthesizers, such filters are essential components for implementing agile radio beams as are widely used, in particular, in applications where it is desirable to be able to change the transmission frequency of a signal very quickly.
Such agile filters must not only be capable of changing center frequency very quickly, they must also be capable of withstanding varying power levels:
when used for transmission, they operate under high power (at the output from transmission amplifiers) and their function is to prevent the transmitter polluting adjacent channels;
when used as channel filters on reception, they enable the receiver to amplify only the wanted signal and to exclude pollution from adjacent transmissions; although they are then theoretically operating at low power, they must nevertheless avoid being damaged or destroyed by said adjacent transmissions which may apply disturbing signals thereto at power levels that are much higher than those of normally-received signals.
Numerous types of agile microwave filter are already in use or are described in the literature.
The most conventional agile filter is a filter having resonant cavities that are tuned mechanically under motor control. This type of filter can withstand high powers, but it is no longer suitable for present-day applications since its operating time to change frequency is extremely long, e.g. of the order of one minute, whereas certain projected applications require a channel switching time of much less than one second, and more specifically of the order of a millisecond. In addition, they suffer from other drawbacks of being bulky, relatively heavy, and particularly expensive and complex to manufacture because of the difficult and accurate mechanical constraints that must be satisfied.
A resonant cavity may also be tuned by using variable capacitance diodes or "varactors" which react in a very short period of time, typically of the order of a microsecond. Unfortunately such filters cannot be used with radio beams because the power they can accept is much too low (less than 0 dBm), and in addition their operating frequency is limited with presently available components to frequencies below 2 GHz.
Such tuning may also be obtained quickly by the action of a polarizing magnetic field on a resonator having yttrium iron garnet (YIG) beads. Nevertheless, the power performance of such a resonator is much too low, being of about the same order of magnitude as for the above-mentioned varactor filters.
Patent Document FR-A-2 521 786, which refers to Document FR-A-2 509 537 describes a bandpass filter having dielectric resonators placed in a waveguide having a cross-section whose dimensions are about 2.5 times the transverse dimensions of the resonators. A YIG pellet is placed on each of the dielectric resonators so as to make the filter magnetically tuneable under the action of adjustable external magnetic fields. The air-gap in the magnetic circuits for creating these magnetic fields is then very large such that in order to obtain a satisfactory tuning range the number of ampere-turns required is very large and current consumption is therefore very high. In addition, making such composite resonators is relatively expensive, specifically because of their composite nature.
Finally, Document FR-A-2 610 766 in the name of the present applicant describes a power resonator constituted at least in part by polycrystalline ferrite and in which tuning is changed very quickly by applying an adjustable magnetic field. The technique disclosed in that document is based on using a resonant coaxial line made from a cylindrical bar of metal-plated polycrystalline ferrite. This ferrite resonator is placed in a device suitable for generating a variable magnetic field. The field thus causes the magnetic permeability of the ferrite material to vary in such a manner as to vary the electrical length of the coaxial line, thereby varying the frequency of the resonator.
However, the above technique does not enable high unloaded Q-factors to be obtained as are essential for making the very narrow band microwave filters required by present radio beams. The unloaded Q-factor of the structure described in said Document FR-A-2 610 766 is limited by the metallic confinement of the microwave electromagnetic fields of the coaxial cavity, and this limitation is made worse by the dielectric permittivity of the material reducing the dimensions of the resonant cavity compared with a corresponding cavity filled with air, and also by the appearance of interfering parasitic modes which would be the consequence of enlarging the structure for the purpose of increasing its unloaded Q-factor.
The invention seeks to remedy these various drawbacks.